Forum: Shoring Up the Soil

Retaining walls have hundreds of applications: from residential landscaping to highway construction, from reinforcing channel embankments to making all-but-unusable land suitable for building. The types of retaining walls-segmental, mechanically stabilized earth, cast-in-place-are almost as varied as the uses. They are valuable, age-old tools used not only for construction, but also in the broad context of erosion and sediment control.

Erosion Control asked the following people to participate in a roundtable discussion on how and where different types of retaining walls are best used and what to consider when choosing a wall.

Richard Bodie, general manager, HydroPave Ltd., Grapevine, TX

Barbara French, marketing manager, Keystone Retaining Wall Systems, Minneapolis, MN

Dion Gray, marketing manager, The Reinforced Earth Company, Vienna, VA

Jeff Nelson, marketing and creative manager, Versa-Lok Retaining Wall Systems, Oakdale, MN

Erosion Control (EC): Are there situations when retaining walls are not the best choice for slope stabilization; if so, what might they be? For instance, are there practical limits for the use of retaining walls (slope length, angle, running length, soil types, drainage, and so on)?

Barbara French: Practical height limits of retaining wall structures are probably in the neighborhood of 40-50 ft. For retaining walls to be built to these heights, the land value must be at a real premium or at an extremely desirable location. A sure way to end up with a nightmare project is to use poor or low-strength soils that are less than desirable in a reinforced slope and to place them behind a vertical structure.

Many times the solutions that retaining walls provide (use of otherwise useless space) also pose limits on how much space we can utilize. “Fill walls” or “fill sites” are much easier to accommodate. By pushing the wall out to the desired limit within the property line, we can build a vertical structure. In a “cut wall” or “cut site,” however, the designer might be limited to a 1:1 ratio of wall height to properly line easement to accommodate reinforcement lengths and safe excavation limits.

Jeff Nelson: Practical limits of retaining walls are usually determined by a ratio of height versus value of the property that the wall preserves or creates. Structurally stable segmental walls can be designed and constructed to heights of 50 ft. or greater. As the height of the wall increases, however, so does its cost. At some point, wall cost will become greater than the value of the property preserved-usually encouraging changes in construction plans or selection of a different site.

Dion Gray: It is generally unsuitable to construct standard reinforced earth (RE) walls in locations where the structure becomes submerged in seawater or where groundwater is considered aggressive. Special coatings, however, may be applied to the steel reinforcing strips to resist metal loss even in aggressive subsurface conditions. The use of RE walls, as with any mechanically stabilized earth (MSE) wall system, assumes that there is sufficient horizontal space to accommodate the reinforcing elements and select fill that make up the overall retaining wall.

Richard Bodie: There are really two limitations for segmented retaining wall products: (1) when there is no allowable horizontal dimension behind the wall to allow for the appropriate geosynthetic reinforcement, (2) when you have very poor supportive subsoils in regard to bearing capacity, because stabilizing those weak-bearing soils is not cost-effective. You could actually build a wall structure on top of poor-bearing-capacity soils provided you then stabilize those soils or use some sort of pier support design, but that would just be too cost-prohibitive.

EC: Reverse the question: Where are retaining walls best used?

Barbara French: Retaining wall applications are best suited for vertical-grade separations where property lines, easements, and right of ways would otherwise be encroached upon by slopes or slopes too steep to reasonably maintain. If important site considerations such as soil strength, site drainage, slope stability, surcharge loading, and property easements are properly addressed, retaining walls are often practical and attractive site solutions for vertical-grade separations from 1 to 50 ft. in height.

Richard Bodie: The primary benefit of segmented retaining walls is that, at heights above 5-6 ft., you’re getting a structure that not only provides the retaining function but also gives you the ability to introduce the architectural aspects of the wall in regard to color, shape, texture, and shadowing. For the same or less cost than conventional cast-in-place structures, you’re getting the same retaining element, but you’re getting more of an architectural feature.

Jeff Nelson: Segmental walls may be designed to accommodate most any type of soil conditions and any type of wall applications-from 40-ft.-tall walls to relatively small landscaping features. Made of concrete, segmental walls are environmentally safe and are also well suited for use along shorelines and waterways. They may be found in residential settings, in commercial environments, around golf courses, and along roadways-just about anywhere a need for earth retention exists. They may be used for erosion control as well as to create more usable building sites. It’s not a case of the product being suitable for larger projects and overengineered for smaller jobs; our segmental walls perform equally well in virtually all project types. Our retaining walls are routinely used by private clients, state transportation departments, and the US Army Corps of Engineers. Remember, we are discussing retaining walls, not foundation walls. Segmental walls are designed to retain soil. They are not intended to support bridges, buildings, or other structures that require rigid footings or footings below frost.

Dion Gray: RE walls are best used as an economical replacement to cast-in-place concrete retaining structures. The savings in concrete and formwork alone make RE walls an excellent system easily constructed without the need for specialty construction crews. Experience with RE walls spans over 30 years with significant research and development to address a wide variety of physical conditions. The highest single RE wall in the US was constructed at the Kennedy Interchange in Georgia and stands 110 ft. tall. RE walls can be designed for any variety of surcharge loads, including spread footing abutment loads, railroad loads, large vehicular loads, and slope surcharges.

EC: Since the category is relatively broad, how do you characterize your product and where its highest and best uses lie?

Dion Gray: RE walls fall under the category of mechanically stabilized systems that use inextensible (steel-strip) reinforcement and precast concrete panel facing. RE walls are an economical way to meet everyday earth-retention needs at highway and bridge separations, waterfronts, airports, loading docks, industrial facilities, and commercial and residential developments. They are also used in response to difficult design conditions, such as restricted space, unstable slopes, and poor foundation conditions. And for structures subject to flooding or other hydrodynamic forces, or those in seismically active areas, the free-draining nature of RE backfill and the natural strength and flexibility of the overall system give designers a powerful way to simultaneously solve difficult stability issues and achieve economy.

Richard Bodie: One of our biggest markets is the erosion control market for open-flow channel embankments. It’s more of a design-driven consideration. You have to have protection at the toe of the wall, and you must use a free-draining structural backfill. The wall products have been tested in laboratory conditions to substantiate that application, more rigorously than for some other applications. You need to protect the toe of the wall so it doesn’t get undermined. You need to use free-draining backfill so the wall doesn’t blow out during rapid drawdown, when the water level exceeds the height of the wall and then drops when the storm event is over. Water is having to move through the structure. A free-draining backfill will readily dissipate that hydrostatic energy coming through the reinforced fill and drainage layer.

Jeff Nelson: The economy of segmental retaining walls is related to two characteristics: they are installed without mortar and they are constructed on shallow, granular pads. Mortarless construction offers several benefits. Walls are easily installed by landscape contractors, grounds maintenance personnel, or municipal construction crews. Without mortar, water is free to weep through wall-face joints to help avoid buildup of hydrostatic pressure. Without mortar or rigid footings, the wall system remains somewhat flexible and can tolerate moderate earth movement, such as that from freeze/thaw cycles, without damage. Also, construction on shallow pads eliminates the need for deep excavations.

EC: What are the major issues of site preparation?

Richard Bodie: First, the soils on-site need to meet the soil parameters used in the design of the wall. When the site-specific soils do not meet the project-design-requirement soils, you have failures-bulging or blowing out of the wall. The second major issue is making sure compaction requirements are met. Backfill materials aren’t always installed according to standard compaction requirements, and that’s primarily because of lack of onsite testing and lack of optimum moisture conditions, particularly in the south where you have very arid environments and sometimes there’s not a water truck on the job. It’s impossible to get appropriate compaction if you don’t have the optimum moisture content in the backfill soils. Third, you need to make sure there is appropriate drainage not only on top of and around the wall, but also within the reinforced backfill section of the wall. That includes water that’s draining above ground and water that’s draining below the grade elevation-both have to be accounted for. If you don’t drain the wall, the soils can become saturated, and you’ll have a structural failure.

Barbara French: Major site preparations include the process of layout-getting correct elevations to match up to site grade alignment and proximity to property lines. Next, excavation of the zone behind the wall is necessary in most designs to be ±60% of the total wall height. In some sites with poor soils (low friction angle) or with steep slopes above the wall, geogrid lengths may exceed a 1:1 relationship. The amount of vertical layer is dependent on grid strength and soil properties, but a good rule of thumb is one course every 2 ft. vertical or for a distance equal to two times the block depth (front to back). The next step is the foundation trench preparation (approximately 1-2 ft. deep x 2 ft. wide). Use roadbase, crushed stone, or lean concrete to create a leveling pad. Next, start setting units. Backfill and compaction are critical to meet 95% standard Proctor compaction.

Dion Gray: The major issues in site or foundation preparation are excavation, drainage, leveling-pad construction, and wall-alignment setup. The site is initially excavated to the depth and width specified on the plans for the length of the wall section to be built. All unsuitable material (deleterious matter, trash, and soft soils) is removed and replaced, as necessary, with compacted fill as directed by the engineer. If required by the specifications, the foundation subgrade is proofrolled and compacted to verify a density suitable to meet the bearing pressure shown on the design plans. Following excavation, runoff is directed away from the area of construction, and drainage systems are installed as necessary in accordance with the project plans and specifications. A nonreinforced, smooth-finish concrete leveling pad is formed and cast at each foundation elevation. Leveling pads have nominal dimensions of 6 in. thick x 12 in. wide and are cast using a minimum 2,000-psi, 28-day compressive-strength concrete. The pads should cure for a minimum of 24 hours before setting panels for the RE wall. The leveling pad is installed for a clean, level working surface and should not be considered a structural foundation element in itself. Once the leveling pad is in place, a chalk line is made to establish a control line for the face of the retaining wall. From this point the first panel course is set and RE wall construction begins.

EC: Are there any standards that apply?

Richard Bodie: There are design standards available. The National Concrete and Masonry Association (NCMA) has published a generic design guideline. It also has a design manual for seismic loading to design the walls to sustain certain levels of seismic loading. Predominantly rigid structures have not done well performancewise.

Jeff Nelson: Engineering design methodology for segmental retaining walls, including geosynthetic reinforced walls, is well established. Governmental agencies, including the American Association of State Highway and Transportation Officials (AASHTO), the Federal Highway Administration, and the NCMA, have developed design guidelines and methodologies for segmental retaining walls.

Barbara French: Standards of practice include American Society for Testing and Materials (ASTM) C1372, Standard Specification for Segmental Retaining Wall Units, published in 1999; NCMA-SRW (Segmental Retaining Wall) design standards; International Conference of Building Officials (ICBO) Evaluation Report 4599, an ICBO guide to evaluation and product use on projects in its jurisdiction and specific to the Keystone retaining wall system; the Highway Innovative Technology Evaluation Center Federal Highway Design Evaluation Process; and international design standards for mechanically stabilized earth structures from Britain, Hong Kong, and Australia.

Dion Gray: The components of the RE wall system-precast concrete panels, steel reinforcing strips, bolt sets, bearing pads, rubber shims, filter cloth, adhesive, and tie strips-typically meet either AASHTO or ASTM standards. Construction and quality-control procedures are governed by state or local requirements and are in accordance with the manual prepared by the Reinforced Earth Company that accompanies construction plans for our RE walls.

EC: What makes your retaining walls special?

Richard Bodie: We market, sell, manufacture, and distribute Anchor Wall Systems. From a constructability standpoint, they have connectors that are integral to the block; they’re not an accessory that must be installed as an adjunct to the system. All of our systems have integral concrete connectors that provide horizontal alignment, structure dimensional setback, and additional shear-strength resistance for local block stability. They provide excellent technical support, which is a key to good wall designs.

I think the real issues for the engineers as well as the contractors are that we have a proven system, we have local people who can provide technical support for design and construction issues, and because of our large manufacturing base, we can handle any shortages that occur on the job site. Our local manufacturing capability means that if you need more materials, you don’t have to delay the project.

Jeff Nelson: Versa-Lok retaining walls systems offer a combination of durability and design flexibility unmatched by any other segmental retaining wall system. Our solid concrete units offer unsurpassed durability before, during, and after construction. Unlike other segmental systems, Versa-Lok units have no cores to fill, which eliminates additional materials, unnecessary labor, and uncertainties of improper construction. One standard unit is used to build straight walls, inside corners, outside corners, curves, and steps. Because the standard unit is used for all design elements, no special units need to be ordered or estimated. Typically, Versa-Lok walls offer savings of 25-40% compared to cast-in-place, precast panel, and crib-type structures.

Barbara French: Keystone’s patented pin system and use of unit drainage material in the fascia provide design professionals and contractors with a true retaining wall system. The pins are made of high-strength pultruded fiberglass, which resists deterioration while offering high shear resistance. The pinning mechanism ensures wall alignment, positive mechanical connection to the soil reinforcement, and a true interlock of the concrete masonry units. When the retaining walls are really put to the test under extreme loads and hydrostatic conditions, nothing beats the integrity and security of the Keystone pinned system.

The Keystone family of products offers the marketplace the right-size units for various market needs. From the industry-leading Standard unit at approximately 105 lb. each to the microsize Sedona Stone at approximately 8 lb., Keystone fills the need.

Dion Gray: RE walls are flexible retaining structures that distribute loads over compressible soils and unstable slopes, reducing the need for deep foundations. The RE wall system has high load-carrying capability in both static and dynamic conditions. Its ease and speed of installation are made possible by the prefabricated materials and granular soils used to simplify construction and minimize the impact of bad weather. RE walls have a pleasing appearance with a variety of architectural treatments available. A 15-50% savings is possible in the use of RE walls over concrete walls, depending on height and loading conditions. Finally, RE walls are based on over 30 years of proven performance, more than any other MSE wall system available.